Fuel cell stacks are electrochemical devices that produce water and an electric potential from a fuel, such as a proton source, and an oxidant. Many conventional fuel cell stacks utilize hydrogen gas as the proton source and oxygen gas, air, or oxygen-enriched air as the oxidant. Fuel cell stacks typically include many fuel cells that are fluidly and electrically coupled together between common end plates. Each fuel cell includes an anode region and a cathode region that are separated by an electrolytic membrane. Hydrogen gas is delivered to the anode region, and oxygen gas is delivered to the cathode region. Protons from the hydrogen gas are drawn through the electrolytic membrane to the cathode region, where they react with oxygen to form water. While protons may pass through the membranes, electrons cannot. Instead, the electrons that are liberated from the hydrogen gas travel through an external circuit to form an electric current, which also may be referred to as the electrical output of the fuel cell.
The electrolytic membranes of some fuel cell systems, such as proton exchange membrane (PEM), or solid polymer fuel cell systems, generally need to have a certain level of hydration and to be within a range of suitable operating temperatures in order for the electrolytic membranes to function properly for generation of electrical output. If the membrane is below this range of suitable operating temperatures, the fuel cell will not be able to efficiently produce its electrical output. On the other hand, if the membrane is above this range of suitable temperatures, degradation of the membrane may occur.
During operation of the fuel cell system, a portion of the water for membrane hydration may be generated by the electrochemical reaction of hydrogen and oxygen at the fuel cell cathode. However, additional water is typically required in order to maintain proper membrane hydration. This additional water is often supplied by humidifying the cathode and/or anode gas streams prior to delivery to the fuel cell stack. The relative humidity of these incoming reactant gas streams, which is a ratio of the partial pressure of water in the stream to the vapor pressure of water at the temperature of the stream, impacts the availability of water within the fuel cell stack and thus the hydration of the electrolytic membranes. If too little water is present, the membranes may dry out, leading to a decrease in their proton conductivity, a decrease in their effective area for proton conduction, and/or hot spots that can cause irreversible membrane damage. If too much water is present, the fuel cell stack may flood, leading to a decrease in the availability of reactant gasses to the fuel cell electrodes and a reversible decrease in electrical output.
Conventionally, many fuel cell systems, such as many PEM fuel cell systems, use a humidifier to humidify the cathode air stream that is delivered to the fuel cells of the fuel cell stack, and a stack cooling system that includes a radiator and a coolant pump is used to regulate the temperature and temperature drop of the fuel cell stack by recirculating a heat exchange fluid through the stack in a heat exchange loop. The flow rate of this heat exchange fluid is dictated by the speed of the coolant pump, and the temperature of this heat exchange fluid, which is recirculated through the heat exchange loop, is reduced by the radiator, i.e., by heat exchange with an ambient air stream. In such systems, the humidifier is operated in a feed forward manner to humidify the cathode air stream to a predetermined, or targeted, relative humidity level with respect to the heat exchange fluid that is delivered to the fuel cell stack by the stack cooling system. However, this typically results in the cathode air stream being at a different temperature than the heat exchange fluid that is delivered to the fuel cell stack. This may affect the performance of the fuel cell stack because the membranes of the fuel cells of the fuel cell stack may be at or near the temperature of the heat exchange fluid flowing through the corresponding fuel cells, yet the humidification of the membranes is correlated to the relative humidity of the cathode air stream.